My boyfriend’s best friend is working for the Peace Corp, and is stationed in Tanzania. Does anyone know what the best Calling Card for calling him from our cell phones would be. I need one that has a low rate per minute, but also has reasonable connection rates and doesn’t depreciate over time. Thank you!

I believe that you can use some VoIP services instead of calling cards. I was using ivoipe.com and it has great rates and good quality services.
Visit there site, they have a live chat were you can get help from there agents.